net, internal/poll, net/internal/socktest: set SOCK_{CLOEXEC,NONBLOCK} atomically...

net, internal/poll, net/internal/socktest: set SOCK_{CLOEXEC,NONBLOCK} atomically on NetBSD

NetBSD supports the SOCK_CLOEXEC and SOCK_NONBLOCK flags to the socket
syscall since version 6.0. The same version also introduced the paccept
syscall which can be used to implement syscall.Accept4.

Follows CL 40895

Change-Id: I9e4e1829b0382744c7799f4e58929a53b4e193f7
Reviewed-on: https://go-review.googlesource.com/94295
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>